Methods of Manufacture for Quantum Dot optoelectronic devices with nanoscale epitaxial lateral overgrowth

ABSTRACT

Optoelectronic devices are provided that incorporate quantum dots as the electroluminescent layer in an inorganic wide-bandgap heterostructure. The quantum dots serve as the optically active component of the device and, in multilayer quantum dot embodiments, facilitate nanoscale epitaxial lateral overgrowth (NELOG) in heterostructures having non-lattice matched substrates. The quantum dots in such devices will be electrically pumped and exhibit electroluminescence, as opposed to being optically pumped and exhibiting photoluminescence. There is no inherent “Stokes loss” in electroluminescence thus the devices of the present invention have potentially higher efficiency than optically pumped quantum dot devices. Devices resulting from the present invention are capable of providing deep green visible light, as well as, any other color in the visible spectrum, including white light by blending different sizes and compositions of the dots and controlling manufacturing processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application based on pending application Ser. No.10/933,941, filed Sep. 3, 2004, which claims benefit of U.S. ProvisionalApplication No. 60/500,273, filed Sep. 5, 2003, entitled Quantum DotOptoelectronic Devices (Q-DOD) with Nanoscale Epitaxial LateralOvergrowth (NELOG), the contents of both of which are incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to optoelectronic devices and, morespecifically, to devices incorporating quantum dots that (1) serve asactive layers and (2) facilitate nanoscale epitaxial lateral overgrowthand (3) facilitate methods for manufacture of optoelectronic devices.

BACKGROUND OF THE INVENTION

Semiconductor Light Emitting Diodes, commonly referred to as LEDs, wereintroduced in the 1960's when visible red light was produced usinggallium arsenide phosphide (GaAsP) on a GaAs substrate (Ref: N. HolonyakJr. and S. F. Bevacqua, “Coherent (Visible) Light Emission fromGa(As_(1−x)P_(x)) Junctions,” Appl. Phys. Lett., vol. 1, pp. 82-83,1962.). Over the last four decades significant improvements in LEDtechnology, availability of other semiconductor materials, and,generally, optoelectronic technology have led to more efficient devicesbeing produced over a wider spectrum of visible color.

The illumination produced by LEDs is generated by radiativerecombination of electrons and holes in a semiconductor device,generating light (photons) through the process of electroluminescence.In doped semiconductor material, impurity atoms change the electronbalance, either adding free electrons or creating holes where electronscan migrate. Either of these additions makes the material moreconductive. A semiconductor with extra electrons in the conduction bandis called n-type material; free electrons move in the conduction energyband through the processes of diffusion and drift. A semiconductor withextra holes is called p-type material, since it has extra valenceelectron deficiencies (holes); the holes move in the valence energy bandas positive charges through the processes of diffusion and drift. Aheterostructure LED comprises a section of n-type material and a sectionof p-type material with an active layer in between, sometimes quantumsized, and with electrodes disposed in electrical communication with then and p sections.

Light is produced by double heterostructure and “quantum well” LEDs whenfree electrons from the n-type layer recombine in the active layer withholes from the p-type layer. For every electron that falls from theconduction band to the valence band, there is a possibility of producingone photon, resulting in the illumination. The probability that a photonwill be produced by recombination of a given electron is the internalquantum efficiency of the material. Visible light is only produced whenthe diode is composed of certain materials, so called “wide bandgap”materials, with a direct energy gap in the range of visible light. Untilrecently, it was not possible to use LEDs for general lightingapplications, because general “white” lighting requires a blending ofphotons with several different energies, e.g. red, green, and blue, andthe technology did not exist to make bright blue emitters.

Modern innovations in LED technology have led to the use of III-Vsemiconductor materials to produce high-efficiency LEDs at both ends ofthe visible spectrum. For example, III-arsenide-phosphide (III-AsP)materials have been used since the 1960s to produce yellow to infraredLEDs, and 111-nitride (III-N) materials have been used since themid-1990s to produce blue-green to ultraviolet LEDs. [Ref: ShujiNakamura and Gerhard Fasol, The Blue Laser Diode, Springer, Berlin(1997)] The most efficient LEDs are made from double heterostructures,with an extremely thin “quantum sized” layer of light emitting alloysandwiched between larger-bandgap and thicker p-type and n-type layers.The active layer in such devices is commonly referred to as the “quantumwell” and is strictly defined as a one-dimensional (1D) potential wellfor electrons and holes whose width is of order the same or thinner thanthe free-exciton Bohr radius. In a true quantum well, electrons from then-type layer and holes from the p-type layer exhibit 1D confinement,being localized in the quantum dimension, and forming essentially2-dimensional (2D) wavefunctions in the quantum well.

III-AsP device heterostructures are typically grown epitaxially on highquality bulk III-V substrates (e.g. GaAs) and the crystal quality in theactive layers is very good, with on the order of 1000 crystaldislocations per square centimeter (cm) or less. As such, in III-AsPdevices the electron-hole wavefunctions are truly 1D confined, aspreviously discussed. FIG. 1 provides a cross-sectional view of aIII-AsP optoelectronic device, in accordance with the prior art. Thedevice 10 includes a substrate 12 that is formed of gallium arsenide(GaAs), a n-type conductive layer 14 that is formed of n-type aluminumindium gallium phosphide (AlInGaP), a quantum well or active layer 16that is formed of indium gallium phosphide (InGaP) and a p-typeconductive layer 18 that is formed of p-type AlInGaP. This device isexemplary of a red LED heterostructure with 1D confinement.

III-Nitride (III-N) device heterostructures are typically grown onsapphire or silicon carbide (SiC) substrates. Due to lattice and thermalconductivity mismatch between the substrate and the III-Nitride, thecrystal structure in the active layer is low quality, exhibiting up to10⁹ dislocations per square cm. FIG. 2 provides a cross-sectional viewof a III-N optoelectronic device, in accordance with the prior art. Thedevice 20 includes substrate 22 that is formed of sapphire, n-typeconductive layer 24 that is formed of n-type gallium nitride (GaN),quantum well or active layer 26 that is formed of indium gallium nitride(InGaN) and includes high-indium-fraction InGaN quantum dots 28, andp-type conductive layer 30 that is formed of p-type GaN. Quantum-sizedindium-rich dots, or nanoparticles, form spontaneously in InGaN quantumwell layers grown with metal-organic chemical vapor deposition (MOCVD).[Ref: K. P. O'Donnell, R. W. Martin, and P. G. Middleton, Phys Rev Let,82, 237 (1999)] Since the solubility of indium in InGaN is a function ofthe thermodynamic state of the InGaN, quantum dot formation in InGaNactive layers is driven by the variations in thermodynamic state duringthe MOCVD process. The quantum dot formation process can now beempirically controlled to yield efficient light emitting diodewavelengths over a range from about 380 nanometers (nm) to about 520 nm.For wavelengths shorter than 380 nm, the relative lack of indium in theInGaN alloy does not allow for efficient light emitting devices andbeyond 520 nm the InGaN nanostructure does not result in efficientdevice performance.

In the III-Nitride optoelectronic device illustrated in FIG. 2, if theelectron-hole wavefunctions were simply confined in the quantum well asin III-AsP devices, then the III-N devices would not be very efficientbecause the 2D electron and hole wavefunctions would simultaneouslyintersect all the crystal dislocations 32 that act as non-radiativeelectron-hole recombination centers (NRRC) for electrons and holes.However, the InGaN active layer 26 exhibits strong InN-GaN materialsegregation because the layer consists of high-indium fraction InGaNquantum dots 28 in a low-indium fraction InGaN quantum well.Electron-hole pairs are confined in three-dimensions to the smallerbandgap higher-indium InGaN quantum dots and thus do not interact withthe crystal dislocations. Both compositional and quantum-size effectsprovide for the quantum dots to illuminate visible blue light. Thus, itis possible to make high efficiency blue InGaN LED devices in spite ofthe high crystal dislocation densities.

FIG. 3 provides a cross-sectional view of a III-N optoelectronic devicethat exhibits epitaxial lateral overgrowth (ELOG), in accordance withthe prior art. The ELOG process results in significantly lower threadingdislocation density. The device 40 includes a substrate 42 that isformed of sapphire, n-type conductive layer 44 that is formed of n-typegallium nitride (GaN) and includes rows of silicon dioxide (SiO₂)stripes 46. The stripes serve to stop the dislocations 48 emanating fromthe substrate before they propagate into the active layer. Thus, thestripes tend to filter the dislocations and inhibit epitaxial lateralovergrowth of the conductive layer. The device additionally includesquantum well or active layer 50 that is formed of indium gallium nitride(InGaN) and includes high-indium-fraction InGaN quantum dots 52 andp-type conductive layer 54 that is formed of p-type GaN. This structureyields an even more efficient blue InGaN LED device then the exampleprovided in FIG. 2.

While III-AsP and III-Nitride are good materials for high-efficiency redand blue LEDs and laser diodes, neither provides for high-efficiencydeep green devices; i.e. devices that operate in the 555-585 nanometerrange near the peak of the human eye response curve. [Ref: FIG. 6 in A.Bergh, G. Craford, A. Duggal, and R. Haitz, Physics Today, Dec. 2001, p.54] In this spectral region, recently commercialized cadmium selenide(CdSe) quantum dots may provide some illumination entitlement. Recently,significant developments have been made in the deposition of thin layersof CdSe quantum dots onto solid surfaces, assembly of the dots into3-dimensional “quantum dot solids” and incorporation into prototypemicroelectronic devices. For example, CdSe nanoparticles dispersed in apolymer host matrix have been used as a downconverting layer over a blueor ultraviolet LED, see U.S. Pat. No. 6,501,091, entitled “Quantum DotWhite and Colored Light Emitting Diodes”, issued in the name ofinventors Bawendi et al., on Dec. 31, 2002. Such quantum dot phosphordispersions have the property of low optical scattering, since theirsize is significantly smaller than the wavelength of light. CdSe quantumdots have also been shown to be dispersible in an inorganic matrix. See,for example, published United States Patent Application No.2003/0142944, published in the name of inventors Sundar et al., on Jul.31, 2003. In addition, monolayers of CdSe quantum dots have been used asthe active layer of organic LEDs with a 25 percent improvement overprevious QD-LED performance and external quantum efficiency of 0.4percent. [Ref: Coe et al, Nature, 420, 800 (2002)]

Hence, a need exists to develop and manufacture optoelectronic devices,such as LEDs, laser diodes and photodetectors that operate efficiently.In addition, a need exists to extend the wavelengths of light emittingdiodes into the “deep green” range wavelengths near the peak of thehuman eye response curve, i.e. about 555 nm to about 585 nm. Suchdevices and the corresponding methods for producing such devices shouldbe cost-effective and reliable. In addition, the desired devices shouldaccommodate non-lattice matched substrates without having dislocationsin the substrates adversely affect the performance of the devices.

BRIEF SUMMARY OF THE INVENTION

The present invention provides for optoelectronic devices thatincorporate quantum dots as the electroluminescent layer in an inorganicwide-bandgap double heterostructure. Examples of such devices includequantum dot light emitting diodes (QD-LED), laser diodes, photodetectorsand the like. The quantum dots serve as the optically active componentof the device and, in multilayer quantum dot embodiments, facilitatenanoscale epitaxial lateral overgrowth (NELO) in heterostructures havingnon-lattice matched substrates. The quantum dots in such devices will beelectrically pumped and exhibit electroluminescence, as opposed to beingoptically pumped and exhibiting photoluminescence. There is no inherent“Stokes loss” in electroluminescence, thus the devices of the presentinvention have higher efficiency entitlement than optically pumpedquantum dot devices. Devices resulting from the present invention arecapable of providing deep green visible light, as well as, any othercolor in the visible spectrum, including white light by blending thesize of the dots and controlling manufacturing processes. In addition tothe devices, the present invention also provides for novel means ofmanufacturing optoelectronic devices that incorporate quantum dots.

In the present invention the semiconductor quantum dots are disposedbetween two semiconductor electrodes, where the electrode bandgap islarger than that of the dots themselves, thus facilitating (1) directelectrical excitation of the quantum dots, potentially eliminating“Stokes losses” and (2) recombination of electron-hole pairs in aquantum confined environment, maximizing quantum efficiency. The presentinvention is superior to placing quantum dots on top of an LED, becausesuch an optically excited approach results in “Stokes loss” every time aphoton is downconverted. The present invention is also superior toplacing the quantum dots between organic electrodes because organicelectrodes are fundamentally less stable than the inorganicsemiconductor electrodes proposed in the present invention.

In one embodiment of the invention a quantum dot optoelectronic deviceis defined that includes, a layer of a first conductive type, a quantumdot layer disposed on only a portion of the first layer such that otherportions of the first layer remain uncovered by the quantum dot layer,and a second layer of second conductive type that is different from thefirst conductive type disposed on the quantum dot layer and the firstlayer. The device will typically include a substrate having the layer offirst conductive type disposed on the substrate. However, in someembodiments the substrate may be removed after processing. The substratemay be formed of any suitable semiconductor or electrical insulatormaterial, including sapphire, silicon, silicon dioxide, glass siliconcarbide, lithium niobate, lithium gallate, gallium nitride, aluminumnitride, aluminum gallium nitride, zinc oxide or the like. Additionally,the device may include an encapsulation layer disposed over the quantumdot layer and under the second layer.

The first and second layers are typically formed of semiconductormaterials having a bandgap wider than the bandgap of the quantum dots.In one embodiment the first and second layers are formed from a IIInitride conductive type material, such as n-type or p-type galliumnitride. The optional encapsulation layer is typically formed of anon-conductive insulator material, typically the non-conductive versionof the material used to form the first and second layers.

The quantum dot layer is typically a monolayer of quantum dots, althoughin alternate embodiments multiple layers of quantum dots may beimplemented. The quantum dot layer is formed of a material chosen fromthe II-VI group semiconductor compounds. In one embodiment of theinvention the quantum dot layer is formed of quantum dots having aninner core of a first II-VI group semiconductor compound and an outercore of a second II-VI group semiconductor compound, such as an innercore of cadmium selenide (CdSe) and an outer core of zinc sulfide (ZnS).Such CdSe core ZnS shell quantum dots with size 2-6 nm can be used toprovide colors throughout the visible spectrum, while dots of varyingsizes may be blended to provide for white light. The quantum dots may bepatterned on the first layer or otherwise prearranged to providenucleation sites for the second layer and to inhibit nanoscale epitaxiallateral overgrowth.

In an alternate embodiment, a quantum dot optoelectronic device isdefined. The device includes a first layer of a first conductive typehaving a pitted surface, a plurality of quantum dots disposed on thepitted surface of the first layer such that the quantum dots aregenerally located proximate pit openings in the surface of the firstlayer and a second layer of a second conductive type that is differentfrom the first layer disposed on the quantum dots and on the firstlayer. The device will typically include a substrate having the layer offirst conductive type disposed on the substrate. However, in someembodiments the substrate may be removed after processing. The substratemay be formed of any suitable semiconductor or electrical insulatormaterial, including sapphire, silicon, silicon dioxide, glass siliconcarbide, lithium niobate, lithium gallate, gallium nitride, aluminumnitride, aluminum gallium nitride, zinc oxide or the like. The devicewill typically further include an encapsulation layer disposed betweenthe plurality of quantum dots and the second layer.

The first and second layers are typically formed of semiconductormaterials having a bandgap wider than the bandgap of the quantum dots.In one embodiment the first and second layers are formed from a IIInitride material, such as n-type or p-type gallium nitride. The pits inthe surface of the first conductive type layer may be etch pits or anyother type of pits, cavities or pores formed in the surface of thelayer. The pit locations may be correlated with the locations ofthreading dislocations. The pits may also include field emitterstructures that serve to provide cathode luminescence to the device.

The plurality of quantum dots may be made to migrate toward the pitopenings when disposed on the surface of the first conductive typelayer. As such, the quantum dots may be defined as being proximate thepit openings. Similar to the previous embodiments, the plurality ofquantum dots is formed of a material chosen from the II-VI groupsemiconductor compounds. In one specific embodiment the plurality ofquantum dots will be formed of an inner core of II-VI groupsemiconductor compound and an outer shell of another II-VI groupcompound. The dot size will dictate the emission wavelength and, thus,the color of the light emitted. Embodiments having quantum dots ofvarying size will emit white light.

In another embodiment of the invention a method for making a quantum dotoptoelectronic device is defined. The method includes the steps ofdisposing a first layer of a first conductive type on a substrate,disposing a quantum dot layer on only a portion of the first layer suchthat other portions of the first layer remain uncovered by the quantumdot layer, and disposing a second layer of a second conductive type thatis different from the first type on the quantum dot layer and the firstlayer. Additionally, the method may include the step of disposing anencapsulation layer between the quantum dot layer and the second layer.

Disposing the first layer may entail growing a first conductive typelayer by metal-oxide chemical vapor deposition (MOCVD). Highertemperature and therefore more rapid processes can be implemented atthis stage because the quantum dots have not yet been deposited. Thequantum dots may be disposed in solution or slurry form, such as bydrop-cast or spin-coat processing. Additionally, the quantum dots may bedisposed by chemically attaching (i.e., self-assembled MOCVD or MBEprocesses) the quantum dot layer to the n-type semiconductor materiallayer. It is also possible to dispose the quantum dots in a poroussolid-matrix, such as a sol-gel matrix.

Once the quantum dots have been disposed, subsequent processing isperformed at a lower temperature to preserve the stability of thequantum dots. For example, the encapsulation layer may be disposed bygrowing the layer by molecular beam epitaxy at or below 500 degreesCelsius. Additionally, the second layer may be grown by MBE or by OrganoMetallic Vapor Phase Epitaxy (OMVPE).

An additional embodiment of the invention is defined by a method formaking quantum dot optoelectronic devices. The method includes the stepof disposing a first layer of a first conductive type on a substrate,providing for a plurality of pits in a surface of the first layer,disposing a plurality of quantum dots on the surface of the first layersuch that the quantum dots are generally located proximate the pluralityof pits in the surface of the first layer and disposing a second layerof a second conductive type that is different from the first conductivetype on the plurality of quantum dots and the first layer. The pitsprovided in the first conductive type layer can be formed by wet etchprocessing or any other suitable semiconductor processing technique.

Thus, the present invention provides for optoelectronic devices thatincorporate quantum dots as the electroluminescent layer in an inorganicwide-bandgap heterostructure. The quantum dots serve as the opticallyactive component of the device and, in multilayer quantum dotembodiments, facilitate nanoscale epitaxial lateral overgrowth (NELOG)in heterostructures having non-lattice matched substrates. The quantumdots in such devices will be electrically pumped and exhibitelectroluminescence, as opposed to being optically pumped and exhibitingphotoluminescence. There is no inherent “Stokes loss” inelectroluminescence thus the devices of the present invention havehigher efficiency than optically pumped quantum dot devices. Devicesresulting from the present invention are capable of providing deep greenvisible light, as well as, any other color in the visible spectrum,including white light by blending the size of the dots and controllingmanufacturing processes.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is cross-sectional view of a III-AsP heterostructure,optoelectronic device, in accordance with the prior art.

FIG. 2 is a cross-sectional view of a III-N heterostructure,optoelectronic device incorporating quantum dots in the active region,in accordance with the prior art.

FIG. 3 is a cross-sectional view of a III-N heterostructure,optoelectronic device having quantum dots as the active region anddislocation blocking mechanisms in the first conductive layer, inaccordance with the prior art.

FIG. 4 is a cross-sectional view of an optoelectronic device having aquantum dot layer as the active region, in accordance with an embodimentof the present invention.

FIG. 5 is a cross-sectional view of an optoelectronic device havingmultiple quantum dot layers as the active region, in accordance with anembodiment of the present invention.

FIG. 6 is a cross-sectional view of an optoelectronic device havingquantum dots in the active region that are located proximate theopenings of pits in the surface of the first conductive type layer, inaccordance with an embodiment of the present invention.

FIG. 7A-7E are cross-sectional views of various stages in the method formaking optoelectronic devices having quantum dots as the active region,in accordance with an embodiment of the present invention.

FIGS. 8A-8E are cross-sectional views of various stages in an alternatemethod for making optoelectronic devices having quantum dots as theactive region, in accordance with an embodiment of the presentinvention.

FIG. 9 is a graph of the photoluminescence provided by CdSe QDs on n-GaNdevice, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

The present invention provides for optoelectronic devices thatincorporate quantum dots as the electroluminescent layer in an inorganicwide-bandgap heterostructure. Examples of such devices include quantumdot light emitting diodes (QD-LED), laser diodes, photodetectors and thelike. The quantum dots serve as the optically active component of thedevice and, in multilayer quantum dot embodiments, facilitate nanoscaleepitaxial lateral overgrowth (NELOG) in heterostructures havingnon-lattice matched substrates. The quantum dots in such devices will beelectrically pumped and exhibit electroluminescence, as opposed to beingoptically pumped and exhibiting photoluminescence. There is no inherent“Stokes loss” in electroluminescence thus the devices of the presentinvention have higher efficiency than optically pumped quantum dotdevices. Devices resulting from the present invention are capable ofproviding deep green visible light, as well as, any other color in thevisible spectrum, including white light by blending the size of the dotsand controlling manufacturing processes. In addition to the devices, thepresent invention also provides for novel means of manufacturingoptoelectronic devices that incorporate quantum dots.

FIG. 4 provides a cross-sectional view of an optoelectronic deviceincorporating quantum dots, in accordance with an embodiment of thepresent invention. The device 100 includes a substrate 102, a firstconductive layer 104 of a first conductive type, a quantum dot layer 106and a second layer 108 of a second conductive type that differs inconductivity from the first layer. In addition, the device willtypically include an encapsulation layer 110 that encapsulates thequantum dot layer.

The substrate 102 may be formed of any suitable semiconductor material,for example the substrate may be formed of sapphire, silicon carbide orthe like. The substrate should generally be optically transparent to thelight that will be generated by the active QD layers, although this isnot absolutely required, since a reflector (e.g. discrete Braggreflector, DBR) can in principle be incorporated between substrate anddevice heterostructures to reflect light away from a non-transparent,and hence absorbing, substrate. The substrate should be thick enough soas to be mechanically stable through the growth process. For GaNepitaxial growth with MOCVD, a typical substrate is a commercialsapphire wafer with about 250 micrometers of thickness. Typically, afterLED device heterostructures are grown and LED devices are fabricated,the sapphire substrate is thinned down to about 75 micrometers.

The first layer 104 is disposed on the substrate 102. It is noted thatthe term “disposed on” does not necessarily require that the layer orelement be formed directly on the underlying, overlying or adjacentlayer or element and, as such, intermediary layers may exist. The firstlayer will comprise an n-type conductive layer or a p-type conductivelayer as dictated by the design of the device. The first layer willtypically comprise a transparent material having a wide-bandgap, furtherdefined as having a band-gap wider than the bandgap of the quantum dots.Wide-bandgap materials will ensure low absorbance for light emittingdevices, and also insure that electron and holes remain confined in theactive layer to enhance radiative efficiency. For example, the firstconductive layer may comprise a III-nitride semiconductor material, suchas gallium nitride (GaN), zinc sulfide (ZnS), zinc selenide (ZnSe),cadmium sulfide (CdS), zinc oxide (ZnO), magnesium oxide (MgO) and thelike. In one embodiment of the invention the substrate will comprisesapphire and the first conductive type layer will comprise GaN, eitherp-type GaN or n-type GaN. The thickness of the first layer should besufficient to allow low-loss current spreading to the entire activelayer in the resulting LED device. For example, about 3 to about 5micrometers of n-GaN is typically a sufficient thickness in commercialInGaN LED heterostructure LEDs. The actual optimum thickness depends onthe conductivity of the material used.

In alternate embodiments of the invention, the substrate 102 may beremoved after processing, typically after the first layer 104 has beengrown on the substrate. For example, a thick layer of Halide Vapor PhaseEpitaxy (HVPE) GaN may be grown on a sapphire substrate and thesubstrate may be subsequently removed by a lift-off process, such as alaser lift-off process. After the substrate 102 is removed, the firstlayer 104 may be referred to as the device's “substrate”. Thus, in theexample presented, the resulting device may be referred to as having a“GaN substrate”.

The quantum dot layer 106, which may be characterized as a monolayer ora multilayer, is disposed on only a portion of the first layer 104 suchthat other portions of the first layer remain uncovered by the quantumdot layer. Partial surface coverage by the quantum dot layer providesnucleation sites for subsequently disposed second layer nucleation andfor epitaxial lateral overgrowth. Preferentially, the quantum dots maybe located proximate threading dislocations 112 in the first conductivetype layer. In instances in which the quantum dots are located proximatethe threading dislocations crystal quality may be improved afternanometer-scale epitaxial overgrowth since the quantum dots essentiallystop further propagation of the threading dislocations into the secondlayer. A predetermined percent of surface coverage and a predeterminedsurface pattern of quantum dots will typically be chosen to optimize theelectroluminescence attained from resulting devices. For example, in oneparticular embodiment the quantum dot layer will result in cluster-likeformations of quantum dots with each cluster ranging from individualquantum dot size (a few nanometers) to a few micrometers, with spacingin the same range, and with 5 to 95% surface coverage.

The quantum dots 106 are typically formed of a II-VI group semiconductorcompound; such as cadmium selenide (CdSe), cadmium telluride (CdTe),zinc sulfide (ZnS), zinc selenide (ZnSe) or the like. In one specificembodiment of the invention, the quantum dots are comprised of an innercore and another core, both of which may be formed of different II-VIgroup materials. For example, CdSe may form the inner core of thequantum dots and ZnS may form an outer core of the quantum dots. Theouter core serves to stabilize the quantum dots for certainapplications. The size of the quantum dots will dictate the color of theluminescence provided by the dots. For example, CdSe/ZnS core-shellquantum dots having a size in the range of about 3 nm to about 4 nm willemit in the green (i.e., emission wavelengths in the range of about 555nanometers to about 585 nanometers), while other uniform sized quantumdots will provide red and blue luminescence. A blended formulation ofquantum dots of varying size will provide for white luminescence.

The optoelectronic device 100 of the present invention will typicallyincorporate an encapsulation layer 110 disposed on or about the quantumdot layer 106 and the exposed portions of the first layer 104. Theencapsulation layer will typically be formed of a non-conductive,insulating material and in most embodiments the encapsulation layer willbe formed of the non-conductive host material that is common to thefirst and second layers. For example, in embodiments in which the firstlayer is formed of p-type or n-type GaN, the encapsulation layer may beformed of non-conductive GaN. The encapsulation layer will typically bea thin layer, in the range of about 1 nm to about 100 nm, which servesto encapsulate the quantum dots. The purpose of the encapsulation layeris to mechanically stabilize the QDs for subsequent process steps, andalso to prevent current paths that do not flow through the QDs. In thiscase, there is not a true quantum well in the GaAsP sense, but rather adistribution of e.g. CdSe quantum dots in a matrix of wider band-gape.g. i-GaN material. In principle, it should be possible to encapsulatethe quantum dots by formation of the second layer 108, as such, in thoseembodiments a separate encapsulation layer would not be required. In thecase of CdSe QD between n-GaN and p-GaN layers, this approach shouldwork, since the resulting structure is still a distribution of e.g. CdSeQD in a matrix of wider bandgap material, e.g. between p-GaN and n-GaN.The second layer 108 will be disposed on the quantum dot layer 106 andthe first layer 104. In those embodiments in which the second layerserves as the quantum dot encapsulator, the second layer will generallybe formed directly on the quantum dots and the exposed portions of thefirst layer. In those embodiments in which a separate encapsulationlayer is provided, the second conductive layer will generally be formeddirectly on the encapsulation layer 110. It is noted that the secondlayer will differ in terms of conductivity from the first layer. Forexample, if the first layer is an n-type layer, then the second layer isa p-type layer and vice versa.

Similar to the first layer, the second layer will typically comprise atransparent material having a wide-bandgap, further defined as having aband-gap wider than the quantum dots. For example, the second layer maycomprise a III-nitride semiconductor material, such as gallium nitride(GaN), or gallium arsenide (GaAs), zinc sulfide (ZnS), zinc selenide(ZnSe) and the like. Typically, the second layer will be formed of thesame host material as the first layer, differing only in theconductivity type. The second layer will typically be thick enough toallow sufficient current to spread in the resulting LED devicestructures. For example, in a commercial InGaN LED heterostructures, thesecond layer is p-GaN, and will typically have a thickness of about 0.1to about 0.5 micrometers, and the thickness is limited by the currentstate-of-the-art of MOCVD technology for growing p-type GaN.

FIG. 5 is a cross-sectional view of an optoelectronic deviceincorporating multiple layers of quantum dots, in accordance with anembodiment of the present invention. Alternating patterns of quantum dotlayers facilitate nanoscale epitaxial lateral overgrowth (NELOG) inheterostructures on non-lattice matched substrates. As shown themultiple quantum dot layer construct 200 includes a substrate 202, afirst layer 204, a first quantum dot layer 206, a second quantum dotlayer 208, a third quantum dot layer 210 and a second layer 212 of adifferent conductive type than the first layer. Additionally each of thequantum dot layers will typically be encapsulated with a correspondingfirst, second and third encapsulation layer 214, 216 and 218 prior todisposing the subsequent quantum dot layer on the second layer. Each ofthe quantum dot layers will be disposed on a portion iiof the underlyinglayer (i.e., the first layer or a preceding quantum dot layer), so as toallow for nucleation sites to exist and for epitaxial lateralovergrowth. In the illustrated embodiment the substrate may comprisesapphire and the first layer may comprise a III-N semiconductormaterial. Since these materials are not lattice matched, thedislocations are readily apparent. The multiple layers of quantum dotsserve to block the dislocations 220 emanating from the substrate beforethey propagate into the second conductive type layer. Thus, the multiplequantum dot layers tend to filter the dislocations and facilitateepitaxial lateral overgrowth of the conductive layer.

FIG. 6 is a cross-sectional view of an optoelectronic deviceincorporating quantum dots and pits/pits in the first layer, inaccordance with an alternate embodiment of the present invention. Thedevice 300 includes a substrate 302, a first layer 304, a quantum dotlayer 306 and a second layer 308 that differs in conductivity from thefirst layer. In addition, the device will typically include anencapsulation layer 310 that encapsulates the quantum dot layer. Thefirst conductive type layer will be characterized by pits 312 formed orotherwise existing in the surface of the first conductive type layerupon which the quantum dots are disposed. Typically, the pits will beformed by an etch process, although other forms of surface pittingprocesses may also be employed. The pit openings in the surface of thefirst conductive layer will provide for areas where quantum dots willmigrate upon deposition. As such the quantum dots willcharacteristically be proximate the pit openings in the surface of thefirst conductive type layer. Additionally, the pits 312 may includefield emitter structures 314, sharp peaks extending outwardly from theinterior walls of the pits. In such embodiments, application of asufficient electrical voltage across the device will result inelectroluminescence from the first conductive layer 304 in combinationwith cathode luminescence from the field emitters creating an overallrobust and efficient optoelectronic device.

In accordance with another embodiment of the present invention, a methodfor making a quantum dot optoelectronic device is provided. The methodincludes the steps of disposing a first layer on a substrate, followedby, disposing a quantum dot layer on only a portion of the first layerand then disposing a second layer of a conductive type different fromthe layer on the quantum dot layer and the first conductive type layer.Partial coverage by the quantum dots allows for nucleation sites toexist for layer nucleation and for epitaxial lateral overgrowth. FIGS.7A-7E depict cross-sectional views of various stages of the method formaking quantum dot optoelectronic devices, in accordance with anembodiment of the present invention.

FIG. 7A illustrates a cross-sectional view of the first layer 502 havingbeen disposed on the substrate 500. Typically, the first layer is grownon the substrate by conventional metal organic chemical vapor deposition(MOCVD) processing. Conventional MOCVD processing is typically performedat relatively high temperature, higher than 1000 degrees Celsius, whichprovides for fast, efficient growth. In one specific embodiment about 2micrometers to about 10 micrometers of a first layer, such a n-type GaNis grown on a template substrate, such as sapphire. Epi-ready n-GaN onsapphire constructs are commercially available from numerous sources.

FIG. 7B illustrates a cross-sectional view of the quantum dot layer 504having been disposed on a portion of the first layer 502 such that otherportions of the first layer remain uncovered by the quantum dot layer.In accordance with the invention various processes may be performed todispose the quantum dot layer on a portion of the first layer. Prior toquantum dot deposition it may be advisable to perform surface rougheningwith ion milling, MBE or preliminary chemical etching. Such rougheningmay define desired locations for quantum dots. For example, chemicaletching is often used to reveal threading dislocations throughpreferential etching of disordered material. In addition, patterning andliftoff may be used to define the areas of quantum dot concentration andto insure adequate exposure of the first layer after dot deposition.

In one embodiment the quantum dot layer is disposed via spin-on ordrop-cast processing in solution form, such as a solvent based solution.In one specific embodiment, toluene is used as the solvent and thequantum dots are CdSe-core/ZnS-shell in a concentration ranging fromabout 50 micrograms/milliliter to about 200 micrograms/milliliter.Solution parameters such as solvent polarity, vapor pressure andviscosity will be varied to optimize deposition and device performance.The structure is subsequently dried at low temperature to evaporate thesolvent and solidify the location of the quantum dots on the surface ofthe first layer.

In an alternate embodiment the quantum dots may be disposed via spin-onor drop cast processing in slurry form, such as a slurry of toluene andalcohol. In another embodiment the quantum dots and the surface may befunctionalized with chemically reactive groups, and the quantum dotschemically attached to the surface. Alternatively, in another embodimentthe quantum dots may be deposited in a solid matrix such as poroussol-gel. The porous nature of the sol-gel material will allow forsubsequent epitaxial lateral overgrowth.

FIG. 7C is a cross-sectional view of the optoelectronic device constructafter the optional encapsulation layer 506 has been formed on thequantum dot layer 504 and on the exposed areas of the first layer 502.The encapsulation layer is typically a thin layer of a non-conductiveinsulator, typically the non-conductive semiconductor material thatserves as the host material for the first and second layers. Theencapsulation layer may be formed by a low temperature molecular beamepitaxial (MBE) process or any other conventional low-temperatureprocess may be used. Low temperature processing is essential to insurethe stability of the quantum dots and to achieve a peak wavelength inthe range of about 540 nm to about 580 nm (i.e., the dark green range).

FIG. 7D is a cross-sectional view of the optoelectronic device constructin which the second layer 508 is formed on the encapsulation layer 506.The second layer is typically grown on the encapsulation layer by aconventional semiconductor process, such as organometallic vapor phaseepitaxy (OMVPE) or MOCVD. The thickness of the second layer willtypically be generally equivalent to the thickness of the firstconductive layer, for example the thickness may range from about 200nanometers to about 600 nanometers. FIG. 7E is a cross-sectional view ofan alternate method in which the second layer 508 is formed on thequantum dot layer 504 and the first layer 502, absent the encapsulationlayer. In this embodiment of the invention the second layer serves toencapsulate the quantum dots. The second conductive layer is typicallydeposited by a low-temperature process, such as low temperature MBE, toinsure the stability of the quantum dots and to provide a device withthe requisite peak emission wavelength.

FIGS. 8A-8E depict cross-sectional views of various stages of analternative method for making quantum dot optoelectronic devices, inaccordance with an embodiment of the present invention. FIG. 8Aillustrates a cross-sectional view of the first layer 602 having beendisposed on the substrate 600 and after having pits 604, also referredto as pores or cavities, formed in the surface of the first conductivelayer. Typically, the first conductive type layer is grown on thesubstrate by conventional MOCVD processing. In one specific embodimentabout 2 micrometers to about 10 micrometers of a first layer, such as adoped silicon or doped silicon carbide (SiC) is grown on a templatesubstrate, such as sapphire. The pits in the first layer are typicallyformed by a conventional wet etch process. Additionally, the pits may beformed by other conventional semiconductor processing techniques or thematerial itself may be porous. The pits may include field emitterstructures 606 that emit electrons that impinge upon the quantum dots toprovide cathode luminescence to the device.

FIG. 8B illustrates a cross-sectional view of a plurality of quantumdots 608 having been disposed on he first layer 602 such that thequantum dots are located proximate openings of the pits. The pits on thesurface of the first layer will create a generally uneven topography andthe quantum dots will have a general tendency to migrate toward thelower surface levels. As such, the pit openings will generally providefor areas to which the quantum dots will migrate open disposal. Inaccordance with the invention, various processes may be performed todispose the quantum dot layer on the first layer. In one embodiment thequantum dot layer is disposed via spin-on or drop-cast processing insolution form, such as a solvent based solution. In an alternateembodiment the quantum dots may be disposed via spin-on or drop castprocessing in slurry form, such as a slurry of toluene and alcohol. Inanother embodiment the quantum dots may be functionalized and chemicallyattached to the surface. Alternatively, in another embodiment thequantum dots may be deposited in a solid matrix such as porous sol-gel.The porous nature of the sol-gel material will allow for subsequentepitaxial lateral overgrowth.

FIG. 8C is a cross-sectional view of the optoelectronic device constructafter the optional encapsulation layer 610 has been formed about theplurality of quantum dots 608 and on the exposed areas of the firstlayer 602. The encapsulation layer is typically a thin layer of anon-conductive insulator, typically the non-conductive semiconductormaterial that serves as the host material of the first and secondlayers. The encapsulation layer may be formed by a low temperature MBEprocess or any other conventional low-temperature process may be used.

FIG. 8D is a cross-sectional view of the optoelectronic device constructin which the second layer 612 is formed on the encapsulation layer 610.The second layer is typically grown on the encapsulation layer by aconventional semiconductor process, such as organometallic vapor phaseepitaxy (OMVPE) or MOCVD. The thickness of the second layer willtypically be generally equivalent to the thickness of the first layer,for example the thickness may range from about 200 nanometers to about600 nanometers. FIG. 8E is a cross-sectional view of an alternate methodin which the second c layer 612 is formed on the plurality of quantumdots 608 and the first layer 602, absent the encapsulation layer. Inthis embodiment of the invention the second layer serves to encapsulatethe quantum dots. The second layer is typically deposited by alow-temperature process, such as low temperature MBE, to insure thestability of the quantum dots and to provide a device with the requisitepeak emission wavelength.

FIG. 9 provides a graphical representation of the photoluminescence of atest structure precursor of the present invention. The optoelectronicdevice includes a CdSe quantum dot layer disposed on n-type GaNconductive layer and a sapphire substrate. The quantum dots, about 5.4nm in diameter, have been drop cast on the n-type GaN layer via asolution of toluene and subsequently dried at 60 degrees Celsius. Thefirst line 700 illustrates the photoluminescence exhibited by the deviceafter the dots have been drop cast and before any further thermaltreatment has occurred. The second line 702 illustrates thephotoluminescence exhibited by the device after approximately 30 minutesin a MBE growth chamber at about 500 degrees Celsius. It is noted thatthe peak wavelength of the quantum dots is initially about 566 nm, andis shifted 20 nm to about 546 nm after the thermal treatment process. Inorder to achieve 566 nm after completion of the MBE process (i.e., afterthe second conductive type layer has been grown), larger diameterquantum dots may be used that have emission in the about 586 nmwavelength range.

Thus, the present invention provides for optoelectronic devices thatincorporate quantum dots as the electroluminescent layer in an inorganicwide-bandgap heterostructure. The quantum dots serve as the opticallyactive component of the device and, in multilayer quantum dotembodiments facilitate nanoscale epitaxial lateral overgrowth (NELOG) inheterostructures having non-lattice matched substrates. The quantum dotsin such devices are electrically pumped and exhibit electroluminescence.There is no inherent “Stokes loss” in electroluminescence thus thedevices of the present invention have higher efficiency than opticallypumped quantum dot devices. Additionally, the devices resulting from thepresent invention are capable of providing deep green visible light, aswell as, any other color in the visible spectrum, including white light.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1-36. (canceled)
 37. A method for making a quantum dot optoelectronicdevice, the method comprising: disposing a first layer of a firstconductivity type on a substrate; disposing a quantum dot layer on onlya portion of the first layer such that other portions of the first layerremain uncovered by the quantum dot layer; and disposing a second layerof a second conductivity type that is different from the firstconductive type on the quantum dot layer and the first layer.
 38. Themethod of claim 37, further comprising disposing an encapsulation layerbetween the quantum dot layer and the second layer.
 39. The method ofclaim 37, wherein disposing a first layer on the substrate furthercomprises growing a first layer by metal-oxide chemical vapor deposition(MOCVD).
 40. The method of claim 37, wherein disposing a quantum dotlayer on only a portion of the first layer such that other portions ofthe first layer remain uncovered by the quantum dot layer furthercomprises disposing a quantum dot layer on only a portion of the firstlayer such that clusters of quantum dots are disposed on the firstlayer.
 41. The method of claim 37, wherein disposing a quantum dot layeron only a portion of the first layer such that other portions of thefirst layer remain uncovered by the quantum dot layer further comprisesdisposing the quantum dot layer in solution form.
 42. The method ofclaim 37, wherein disposing a quantum dot layer on only a portion of thefirst layer such that other portions of the first layer remain uncoveredby the quantum dot layer further comprises disposing the quantum dotlayer in slurry form.
 43. The method of claim 37, wherein disposing aquantum dot layer on only a portion of the first layer such that otherportions of the first layer remain uncovered by the quantum dot layerfurther comprises chemically attaching the quantum dot layer to thefirst layer.
 44. The method of claim 37, wherein disposing a quantum dotlayer on only a portion of the first layer such that other portions ofthe first layer remain uncovered by the quantum dot layer furthercomprises disposing the quantum dot layer in a solid-matrix.
 45. Themethod of claim 37, wherein disposing a quantum dot layer on only aportion of the first layer such that other portions of the first layerremain uncovered by the quantum dot layer further comprises disposingthe quantum dot layer in a solid-matrix of porous sol-gel. 46.-50.(canceled)
 51. A method for making a quantum dot optoelectronic device,the method comprising: disposing a first layer of a first conductivitytype on a substrate; providing for a plurality of pits in a surface ofthe first layer; disposing a plurality of quantum dots on the surface ofthe first layer such that the quantum dots are generally locatedproximate the plurality of pits in the surface of the first layer; anddisposing a second layer of a second conductivity type that is differentfrom the first conductivity type on the plurality of quantum dots andthe first layer.
 52. The method of claim 51, further comprisingdisposing an encapsulation layer between the plurality of quantum dotsand the second layer.
 53. The method of claim 51, wherein disposing afirst conductive type layer on the substrate further comprises growing afirst conductive type layer by metal-oxide chemical vapor deposition(MOCVD).
 54. The method of claim 51, wherein providing for a pluralityof pits in a surface of the first layer further comprises etching aplurality of pits in a surface of the first layer.
 55. The method ofclaim 51, wherein disposing a plurality of quantum dots on the surfaceof the first layer such that the quantum dots are generally locatedproximate the plurality of pits in the surface of the first layerfurther comprises disposing the plurality of quantum dots in solutionform.
 56. The method of claim 51, wherein disposing a plurality ofquantum dots on the surface of the first type layer such that thequantum dots are generally located proximate the plurality of pits inthe surface of the first layer further comprises disposing the pluralityof quantum dots in slurry form.
 57. The method of claim 51, whereindisposing a plurality of quantum dots on the surface of the first layersuch that the quantum dots are generally located proximate the pluralityof pits in the surface of the first layer further comprises chemicallyattaching the plurality of quantum dots to the first layer.
 58. Themethod of claim 51, wherein disposing a plurality of quantum dots on thesurface of the first layer such that the quantum dots are generallylocated proximate the plurality of pits in the surface of the firstlayer further comprises disposing the plurality of quantum dots in asolid-matrix.
 59. The method of claim 51, wherein disposing a pluralityof quantum dots on the surface of the first layer such that the quantumdots are generally located proximate the plurality of pits in thesurface of the first layer further comprises disposing the plurality ofquantum dots in a solid-matrix of pitted sol-gel. 60.-63. (canceled) 64.The method of claim 51, wherein disposing a second layer on theplurality of quantum dots and the first layer further comprises growinga second layer by molecular beam epitaxy.
 65. A method for making aquantum dot optoelectronic device, the method comprising: forming afirst layer of a first conductivity, an upper surface of the first layerincluding openings on dislocations in the first layer, forming a quantumdot layer directly on only a portion of the upper surface of the firstlayer such that quantum dots are in the openings and that other portionsof the upper surface of the first layer remain uncovered by the quantumdot layer, sizes of the openings and the quantum dots being selectedsuch that the openings trap the quantum dots, the dislocations extendingdown through the first layer below the quantum dots; and forming asecond layer of a second conductivity that is different from the firstconductivity directly on the quantum dot layer and on the other portionsof the upper surface the first layer.